Low-noise fuze

ABSTRACT

This invention relates to the radio-proximity type of ordnance fuze in which the presence of a target is detected by the reflection of a radio signal transmitted from the fuze.

Kalmus et a].

[ Aug. 13, 1974 LOW-NOISE FUZE Inventors: Henry P. Kalmus, 3255 O St., N.W.,

Filed:

Washington, DC. 20007; Harold Goldberg, 5910-32nd St., N.W., Washington, DC. 20015; Milton Sanders, 1445 Ferndale Ave., Chicago, 111.

Oct. 6, 1954 Appl. No.: 460,789

US. Cl 343/7 PF, 102/702 P, 343/14 Int. Cl F42c 13/04, G01s 9/24 Field of Search 343/7, 14, 17.5;

References Cited UNITED STATES PATENTS Mercer 343/17.5

2,462,294 2/1949 Thompson 250/3319 2,543,782 3/1951 Kiebert, Jr. 343/14 2,570,295 10/1951 Vantine, Jr. 343/13 2,602,920 7/1952 Rust et a1 343/14 2,640,156 5/1953 Schu1tz..... 250/3319 2,715,185 8/1955 Roeschke.. 250/3673 2,842,764 7/1958 Harvey 343/7 PF Primary Examiner-Ma1co1m Hubler Attorney, Agent, or Firm Edward J. Kelley; Herbert Berl: Saul Elbaum [57] ABSTRACT This invention relates to the radio-proximity type of ordnance fuze in which the presence of a target is detected by the reflection of a radio signal transmitted from the fuze. 4

3 Claims, 4 Drawing Figures BAA/W050 g DETECTOR F-M OSC/LLATOI? AMPL/F/ER 70 F/Rl/VG A CIRCUIT MODULATOR WARHE'AD HEWMIQ 1a 1914 SE? NW 2 I A BALANCED X DETECTOR F-M OSC/LLATOP AMPL/F/El? F/R/A/G A CIRCUIT fiECE/VED 2 l SIG/ML e, MODULATOR LOCAL S/GNAL INVENTORSI Henry F. Ka/mus Harold Goldberg Mi/fon Sanders LOW-NOISE FUZE The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to us of any roy alty thereon.

The present invention provides an improved fuze in which the transmitted signal is frequency modulated with a very small deviation. When the fuze is within range of a target a reflected signal is received that varies slightly in phase from the transmitted signal. When the received signal is mixed in a detector with a portion of the transmitted signal, a firing signal having a frequency practically equal to the modulation frequency or a harmonic thereof is obtained from the output of the detector. Because the frequency of this firing signal is practically constant and practically independent of target position and velocity, it is readily amplified with a narrow-band amplifier.

By selecting a fairly high modulation frequency 500 kilocycles, for example the firing signal amplifier canbe made highly insensitive to noise and to microphonics. Also, by using a high modulation frequency a large number of cycles of the beat frequency at least 100, for example can be obtained even when the time of encounter with the target is very short. This practically eliminates the possibility present in prior-art fuzes that are fired by a single transient voltage that noise or microphonics will cause premature firing.

There are three requirements for fuzes for guided missiles, in particular, that were of relatively minor importance in the World War II antiaircraft shell fuze: 1 the searching beam should have high directivity; (2) the fuze should respond equally well to targets with zero or high relative velocity; and (3) the fuze should respond with a very short time of encounter, preferably 3.3 milliseconds or less. The present invention makes feasible a fuze that, unlike fuzes operating on the wellknown Doppler principle, meets all three of these important requirements. An object of the present invention is to provide a radioproximity ordnance fuze having, in combination, (I)- a search beam of high directivity, (2) sensitivity to targets with zero or high relative velocity, and (3) re sponse with very short encounter times.

Another object is to provide a radioproximity ordnance fuze in which the frequency of the firing signal is practically constant and, within the useful range of the fuze, substantiallyindependent of the relative velocity and distance of the target.

A further object is to provide a frequency-modulated transmitter in which fundamental-frequency amplitude modulation is minimized.

Other objects, aspects, uses, and advantages of the invention will become apparent from the accompanying drawing and from the following description.

FIG. 1 is a block diagram of a fuze system according to the invention.

FIG. 2 is a vector diagram showing how a returned signal e varying in phase with respect to the transmitted signal 2,, can be combined with a portion of the transmitted signal to produce an amplitude-modulated signal e,, from which a signal of the original modulation frequency can be obtained by detection.

FIG. 3 is a diagram of a balanced detector suitable for use with the invention.

FIG. 4 is a diagram of a particular embodiment of the invention in which a klystron oscillator is used and in which the operating conditions of the oscillator are optimized by means of a novel control circuit.

Referring to FIG. 1, numeral 1 designates a frequency modulated oscillator having a center frequency f,,. Oscillator l is modulated at a frequency f, by modulator 2, the deviation of the carrier frequency being kept very small. For purposes of this description it may be helpful to assume that f is 10,000 megacycles and that f is 500 kilocycles; these frequencies are satisfactory, although it will be understood that a wide range of frequencies can be used.

The modulated output of oscillator 1 is radiated by transmitting antenna 3. If a target 5 is within range, a portion of the signal radiated from transmitting antenna 3 is reflected from target 5 and is picked up by receiving antenna 6. The received signal from antenna 6, and also a portion of the transmitted signal from oscillator l, are applied to a balanced detector 7.

As will be more fully explained below, addition of these two signals gives an amplitude-modulated signal, and the modulation frequency f, is recovered in the detector output. The detector output signal is amplified by a narrow-band tuned amplifier 10 to function a firing circuit 11 and a warhead 12.

It will be understood by persons familiar with radio proximity fuzes that firing circuit 11 may be of a wellknown type, preferably comprising a thyratron and a detonator, that triggers when the output signal from detector 7 attains a critical amplitude. It will be apparent that firing circuit 11 and warhead 12 will not function when the fuze is at a great distance from target 5 but that they will function when the fuze gets close enough to the target to receive at antenna 6 a returned signal of sufficient amplitude and proper phase.

The operation of the fuze system shown in FIG. 1 may be better understood upon reference to FIG. 2.

In FIG. 2, the vertical vector e, represents a portion of the transmitted signal. Vector e, represents the received signalwhen the target is at a certain distance. The received signal arrives displaced in phase from the transmitted signal by a phase angle which varies at a rate determined by the modulation frequency f,,,. The frequency deviation of the transmitter is preferably kept so small that within the useful range of the fuze say 100 feet the excursions of vector e from its center position are no greater than degrees (for triangular modulation); vector e, swings back and forth between positions e, and e," at the modulation frequency rate. It will be seen that when e, and e, are combined they produce an amplitude modulated signal (2,.

It will be understood that, with relative motion of fuze and target, e, will go through a maximum and a minimum each time the fuze-to-target distance changes by a quarter wavelength of the carrier frequency f (0.75 centimeter for a 10,000 megacycle carrier). It will also be understood that there will be similar maxima and minima in the output of detector 7 (FIG. 1) for each change in fuze-to-target distance of a quarter wavelength at the modulation frequency f,,,. Skilled persons will have no difficulty in selecting values of f,,,, as well as of f that will insure adequate f,,, output from detector 7 when the moving fuze is within the desired range of the target.

FIG. 3 shows a balanced detector suitable for use as detector 7 in FIG. 1. A portion of the signal from oscillator 1 is applied as a balanced local signal to diodes and 16. There is no output from the detector until and unless, as the result of the proximity of a target, a phase-modulated signal e, is received. When such a signal is received, the detector gives an output at frequency f Balanced detectors of this type are well known in other applications. Use of a balanced detector has the advantage that any residual amplitude modulation in the local signal is balanced out and does not appear in the detector output.

From what has been disclosed above, persons skilled in the radioproximity fuze art will be enabled to construct and utilize embodiments of the invention in the form shown in FIG. 1. In some embodiments, however, operation will be optimized by the addition of a novel control circuit. FIG. 4 shows an embodiment of the in vention that includes such a circuit. The function of this control circuit is to reduce objectionable amplitude modulation (AM) of the transmitted signal to a minimium.

As already explained, in the invention as shown in FIG. 1 the presence of the target is indicated by a returned signal that, when added to a portion of the transmitted signal, produces an amplitude modulated signal that is detected and amplified. This means that amplitude modulation present in the transmitted signal must be kept to a minimum, so that the amount of AM that leaks to the receiving antenna 6 or 66 is less than that developed when a signal is reflected from a target In FIG. 4, the output of a klystron oscillator 51 is fed through waveguide 55 to transmitting antenna 56. A small portion of the output is applied as a local signal to balanced detector tee 61, which includes diodes 62 and 63. When a target is within range, a target signal is reflected to receiving antenna 66 and fed to balanced detector tee 61. The output of balanced detector tee 61 is amplified by narrow-band amplifier 67, which is peaked at the modulation frequency f,,,.

Klystron oscillator 51 is frequency modulated by superimposing a sinusoidal voltage upon the do potential of the repeller 72. This repeller modulation produces not only frequency modulation (FM) but also a certain amount of amplitude modulation (AM). It can be shown that the AM will be a minimum under conditions of centermode operation, by which is meant operation with the d-c repeller voltage adjusted to the center of the repeller-voltage versus output characteristic of the klystron, the a-c modulation being superimposed at this point. It can also be shown that, with centermode operation, there is no amplitude modulation at the fundamental of the modulating frequency; the only amplitude modulation produced under these optimum conditions is at second and higher even harmonics of the modulating frequency and, since amplifier 67 responds only to the fundamental, this even-harmonic AM is not objectionable. It can be shown further that, as the d-c operating point shifts to one side or the other of the centermode, fundamentalfrequency AM appears; but this AM is shifted in phase by 180 degrees when the dc operating point is shifted from one side of the centermode to the other. The desired control can therefore be accomplished by demodulating a sample of the klystron output, comparing the phase of the fundamental component of the demodulation product with the phase of the modulating signal by means of a phase detector, and using the output of this phase detector to reset the d-c operating point as close as possible to the optimum value.

In FIG. 4 single converter tube 52 of the pentagrid type type 6BE6, for example serves three functions: it functions as an oscillator to provide modulation for klystron 51; it provides as will be explained more fully below the control action described above to maintain centermode operation of klystron 51; and it provides self-rectifying action so that it can be operated from an alternating-current plate voltage obtained from power source 53. This combination of functions in a single tube contributes to the compactness, simplicity, and reliability of the fuze. It will be apparent that a direct-current plate supply can be used instead of alternating current power source 53, and that a separate oscillator can be used."

To obtain the centermode control function, a portion of the output of klystron 51 is applied to diode detector 68. The output of detector 68 is applied to narrowband control amplifier 71, which is tuned to modulation frequency f,,,. The output of amplifier 71 is applied to the No. 3 grid of converter 52. It will be understood that with this arrangement a control signal of the modulation frequency will be applied to the No. 3 grid of converter 52 if, and only if, amplitude modulation of this frequency is present in the output of klystron 51.

It will be readily seen that the do operating voltage applied to repeller 72 of klystron 51 is dependent upon the effective d-c resistance with which converter 52 shunts resistor 73. It will be readily understood that this shunting effect of converter 52 will depend upon the amplitude and phase of the control signal applied to the No. 3 grid of converter 52. As indicated above, the fundamental-frequency amplitude-modulation output of klystron 51 will be zero under conditions of optimum d-c repeller voltage, and zero control signal will accordingly be applied to converter 52 under these conditions. Any deviation from this optimum d-c repeller voltage will result in application of a control signal to converter 52. With respect to the signal on grid No. l of converter 52, the control signal applied to grid No. 3 will be either in phase or 180 degrees out of phase, depending on whether the d-c voltage on repeller 72 has shifted above .or below the optimum centermode value. It can be seen that, provided the control signal is applied with proper polarity, it will automatically tend to restore the d-c voltage on repeller 72 to the optimum value.

The fuze system of the invention affords excellent signal-to-noise characteristics. It can be shown that, with a transmitted power of 200 milliwatts and an antenna gain of 36, a target 7 feet square at a distance of feet will yield a IOO-microvolt signal. Even with a reduction of antenna gain to 20 and of transmitted power to 100 milliwatts, the useful signal is about 40 microvolts. Calculations and experience show that signal values of this order exceed the various noise signals encountered in practice.

Inthe embodiments of the invention that have been described above, a signal of a frequency practically equal to the fundamental modulation frequency is taken from the output of the balanced detector and is amplified by a selective amplifier tuned to that frequency. It will readily be understood, however, upon referring again to FIG. 2, that only in special cases will the modulation of the amplitude-modulated signal e,, that is obtained by combining the returned signal 2, with the local signal e,, be free from harmonics of the fundamental modulation frequency. For instance, suppose that the center position of the received signal e vector in FIG. 2, instead of being at right angles to the local signal (the condition most favorable to fundamental-frequency modulation), bears a zero or ISO-degree phase relation to the local signal. Then, as the vector swings back and forth, it will be readily understood that the resulting amplitude modulation will contain mainly even harmonics. Assuming in each case that the center position of the e, vector is optimum, it can be shown that production of the fundamental modulation frequency will be maximized when the limit of phase excursion of the received signal is 103; that for maximum second harmonic the corresponding figure is 178; for third harmonic, 240; and so forth. (These figures are for sinusoidal modulation; for triangular modulation, the corresponding figures are 90,l80, and 270.)

It will therefore be understood that it is possible to employ the invention with a harmonic mode of operation; the amplifier or 67 is tuned to the desired harmonic instead of to the fundamental, and the frequency deviation of the transmitter with modulation is increased to optimize the production of the desired harmonic when the returned signal is mixed with the local signal. The sensitivity of fuzes according to the invention is limited largely by the presence of undesired amplitude modulation at the transmitter, and we have discovered and demonstrated that with some klystrons the ratio of desired-to-undesired amplitude modulation, i.e., the signal-to-noise ratio, is better with harmonic mode operation, using either the third or the fifth harmonic, than with fundamental mode operation. With klystrons, the presence of considerable amounts of even-harmonic amplitude modulation in the klystron output permits the harmonic mode of operation at odd harmonics only.

Assuming a constant frequency deviation at the transmitter, the phase excursion of the received signal will of course vary with fuze-to-target distance. In general, it is desirable to apply a deviation that will maximize production of the desired harmonic when the fuze-to-target distance is approximately the greatest at which detonation is desired. As the fuze approaches the target the phase excursion becomes smaller. A disadvantage of harmonic mode operation lies in the fact that at relatively short distances the harmonic signal may practically disappear, so that with harmonic operation there may be dead zones very near the fuze.

It will be apparent that the embodiments shows are only exemplary and that various modifications can be made in construction and arrangement within the scope of the invention as defined in the appended claims.

We claim:

1. A phase-modulation ordnance fuze comprising: transmitter means for producing and radiating a transmitted signal of radio frequency; means for receiving a received signal, said received signal consisting of a portion of said transmitted signal reflected from a target; means for applying frequency modulation to said transmitted signal; means for combining the received signal with a portion of the transmitted signal to produce an amplitude-modulated signal; detector means for demodulating said amplitude-modulated signal; and selective amplifier means for amplifying a component of the output of said detector, said selective amplifier being peaked at the fundamental modulation frequency, the component thus amplified being utilized to cause detonation when the fuze is within range of a target.

2. The invention according to claim 1 wherein said transmitter means comprises a klystron oscillator, wherein said selective amplifier is peaked at an odd harmonic of the'modulation frequency, and including, in addition, a control means for automatically minimizing fundamental-frequency amplitude modulation in the output of said transmitter means, said control means comprising means for taking a sample of the output of said transmitter means; detector means for obtaining a control signal of the fundamental modulation frequency from fundamental-frequency amplitude modulation present in said sample; and means for automatically applying said control signal to shift the operating conditions 'of said transmitter means in a direction favorable to the production of a lower percentage of fundamental-frequency amplitude modulation.

3. A phase-modulation ordnance fuze comprising: transmitter means for producting and radiating a transmitted signal of radio frequency; means for receiving a received signal, said received signal consisting of a portion of said transmitted signal reflected from a target; means for applying frequency modulation to said transmitted signal; means for combining the received signal with a portion of the transmitted signal to produce an amplitude-modulated signal; detector means for demodulating said amplitude-modulated signal; and selective amplifier means for amplifying a component of the output of said detector, said selective amplifier being peaked at a harmonic of the fundamental modulation frequency, the component thus amplified being utilized to cause detonation when the fuze is within range of a target. 

1. A phase-modulation ordnance fuze comprising: transmitter means for producing and radiating a transmitted signal of radio frequency; means for receiving a received signal, said received signal consisting of a portion of said transmitted signal reflected from a target; means for applying frequency modulation to said transmitted signal; means for combining the received signal with a portion of the transmitted signal to produce an amplitude-modulated signal; detector means for demodulating said amplitude-modulated signal; and selective amplifier means for amplifying a component of the output of said detector, said selective amplifier being peaked at the fundamental modulation frequency, the component thus amplified being utilized to cause detonation when the fuze is within range of a target.
 2. The invention according to claim 1 wherein said transmitter means comprises a klystron oscillator, wherein said selective amplifier is peaked at an odd harmonic of the modulation frequency, and including, in addition, a control means for automatically minimizing fundamental-frequency amplitude modulation in the output of said transmitter means, said control means comprising means for taking a sample of the output of said transmitter means; detector means for obtaining a control signal of the fundamental modulation frequency from fundamental-frequency amplitude modulation present in said sample; and means for automatically applying said control signal to shift the operating conditions of said transmitter means in a direction favorable to the production of a lower percentage of fundamental-frequency amplitude modulation.
 3. A phase-modulation ordnance fuze comprising: transmitter means for producting and radiating a transmitted Signal of radio frequency; means for receiving a received signal, said received signal consisting of a portion of said transmitted signal reflected from a target; means for applying frequency modulation to said transmitted signal; means for combining the received signal with a portion of the transmitted signal to produce an amplitude-modulated signal; detector means for demodulating said amplitude-modulated signal; and selective amplifier means for amplifying a component of the output of said detector, said selective amplifier being peaked at a harmonic of the fundamental modulation frequency, the component thus amplified being utilized to cause detonation when the fuze is within range of a target. 